Traditional protein labeling reactions for capillary electrophoresis (CE) with laser-induced fluorescence (LIF) detection suffer from a variety of disadvantages. The reactions can be nonquantitative on a reasonable time scale, require relatively high concentrations of protein and fluorophore, and can give multiple reaction products that can not be separated. Herein, we describe a new noncovalent labeling technique that is rapid, selective for myoglobin, and gives a simple reaction product. Myoglobin is denatured with either 5.4 m urea or low pH (2.0). The denatured myoglobin releases its nonfluorescent heme group. A fluorescent porphyrin (protoporphyrin IX (PPIX) or its zinc (II) complex, Zn-PPIX), is added to the mixture and the solution conditions are altered (dilute to 0.54 m urea or adjust pH to 7.0) to allow myoglobin refolding. Upon refolding, the protein incorporates PPIX from solution, thus making the reaction product fluorescent. The experimental conditions have been optimized for both urea and low-pH denaturation of myoglobin. The latter procedure produces a detection limit of 50 nm. Alternatively, the reaction can be performed without denaturation by a simple exchange of the porphyrins. The use of Zn-PPIX yields the most efficient reaction. The low-pH reaction is unaffected by a 2000-fold excess of bovine serum albumin.